This invention relates to a cryogenic temperature storage chamber and, more particularly, to a laboratory freezer appliance that provides a freezer storage space, e.g., on the order of 5 to 20 cubic feet, capable of storage temperatures of -160.degree. C. and lower.
In both research and diagnostic laboratory applications, low temperature refrigeration of living biological systems and biomaterials is required to produce satisfactory preservations. That is, the biochemical and physical processes by which biomaterials sustain life are affected to varying degrees by temperature. Thus, in applications where ultralow temperatures are successful in arresting these processes, lower storage temperatures are desired to achieve more satisfactory results, particularly for long term storage of biological specimens. The need therefore exists for a reliable laboratory freezer appliance which provides a usable freezer storage space at a consistent and uniform ultralow temperature, e.g., -160.degree. C. and lower, for essentially unattended, extended storage periods.
There are two types of equipment which currently attempt to address, in part, this need. One is by stored refrigeration in the form of vacuum insulated liquid nitrogen dewars designed with a storage space in the vapor above the liquid nitrogen. There are a number of limitations to liquid nitrogen dewars. First, the only practical insulation is in the form of vacuum insulation. Due to strength requirements, the configuration of the storage chamber must necessarily be either cylindrical or spherical which is not an efficient use of space in traditional rectangular buildings and rooms. Second, nitrogen is a liquid at -196.degree. C. at atmospheric pressure, which is an acceptable storage temperature. However, in liquid nitrogen dewars, the temperature may vary greatly, for example, up to 100.degree. C. from top to bottom depending on the design of the vessel and the quality of the insulation, with a significant portion of the chamber maintaining temperatures much warmer than the desired -160.degree. C. temperature. Thus, the physical placement of specimens within the vapor dictates their long term storage temperature, and uniformity and repeatability of storage conditions, particularly over long storage times, is as a practical matter impossible. Third, the source of cooling in a liquid nitrogen dewar is the phase change of the nitrogen from liquid to vapor. Thus, it is necessary for the dewar to regularly receive a fresh supply of liquid nitrogen to replace the boiled-off quantity. Although liquid nitrogen is not particularly expensive, availability and handling do cause problems, and liquid nitrogen cannot be stored indefinitely at ambient temperatures of typically 20.degree. C.
The other type of equipment attempting to provide ultralow refrigeration temperatures is a mechanical system using a mixture of refrigerant components which are compressed by one or more refrigeration compressors. Such refrigeration systems can include a single standard commercial air-conditioning compressor which serves as a pump to move the refrigerant, which is a mixture of fluorocarbon refrigerants, through the system, an air- or water-cooled condenser which cools the compressor and removes heat from the refrigerant by partially changing the mixture from vapor to liquid, a liquid/vapor separator which separates liquid refrigerant from vapor and returns lubricating oil to the compressor, multiple heat exchangers to effect the cooling process, and an evaporator coil through which the refrigerant flows at ultralow temperatures to absorb heat from the freezer interior and deliver it to the condenser for removal. Again, there are a number of problems with this refrigeration system. First, these systems currently operate at temperatures of -135.degree. C. to -150.degree. C. which fall short of the desired temperature of -160.degree. C. Second, the development of the refrigeration circuit for this product is highly intuitive because the properties of the mixed refrigerants are difficult to predict with any accuracy as are the heat transfer and flow characteristics of the mixtures. Third, the mixed refrigerants are fluorocarbon refrigerants which may have to be replaced in the future for environmental reasons.